TNFSF11 and breast cancer: Interestingly, tumor cells cultured in vitro with recombinant RANKL increased IL-6 expression in response to RANKL, and similar results were found with a co-culture of mouse primary osteoblasts with breast cancer cells or conditioned media from breast cancer cells [99,100], suggesting that IL-6 and RANKL form a feed-forward loop in bone-DTCs.